In scintillation cameras, images of radiation fields are formed. In the most common application, scintillation cameras are used for medical diagnostics, in which a medical patient has a small quantity of a given radioactive isotope absorbed into the tissue or tissues to be studied in order to provide an image of such tissue or tissues (planar scans or CAT scans). The nuclear camera has a front focussing screen called a collimator which can be used to allow radiation to pass therethrough without disturbance at only certain given angles. The collimator is often a slab of lead having a large number of passage holes therein. The camera then has a scintillator crystal (such as a thallium (Th) activated sodium iodide (NaI) crystal) which is inside a sealed housing of the camera. When a high energy photon interacts with the scintillator crystal, a very small flash of light is created in the scintillator crystal. An array of photomultiplier tubes (PMT's) act as light detectors of a very high sensitivity to produce electrical signals proportional to the intensity of the light event in the scintillator. The PMT's are optically coupled to the scintillator crystal. The signals from the PMT's must then be processed to determine whether or not the light event was generated by a photon of appropriate energy and what the exact position of the light event was, in order that an image computer may use all the position data points to form a distribution image of the radiation field. In medical diagnostics, this may give an image of a given organ to show any deformity in shape or absence of natural circulation therein, the patient having ingested a substance carrying a radioactive isotope which will be transported in the body to the given organ.
For the quality of the image, it is essential that the PMT's provide accurate signals which are processed to give precise position data of only those events which are valid. It is also important that such an image is formed rapidly so that the time required using the medical diagnostic scanner is reduced without needing to resort to higher concentrations of the radioactive isotopes.
Scintillation events arise from interaction with gamma photons at a very high countrate. Since only a fraction of the scintillation events represent valid events, valid event discrimination is essential for good image production. Valid events are those in which the gamma photon to be detected is emitted by the tissue to be studied in the direction of the scintillator crystal without any intermediate interaction which would result in a loss of energy and a change of direction. In order to determine whether an event is valid or not, a sum of the PMT signals is generated and analyzed in order to determine whether the event has an energy which, within tolerance, is the expected energy of the photons generated by the radiation field. With the high countrate of events taking place in the scintillator (especially when the scintillator has a relatively large surface area or aperture), the sum signal of the PMT's often has overlapping pulses resulting from near simultaneous events.
In the prior art pulse pile-up discrimination systems, it has normally been a goal to ignore those events which occur simultaneously with another event even if the peak of the event indicates that the energy of the event is a valid one. Basically, the prior art systems relate to pulse pile-up (overlapping events) rejection systems in order to eliminate erroneous position data which can arise when either a false determination of an event takes place because the event results from two simultaneous invalid events which when analyzed by the prior art valid event discrimination system can be determined to be valid, or to avoid a wrongful position calculation given that significant signal has been received at the same time from two different points in the scintillator crystal which can sometimes confuse the position calculation computer or circuit.
In the prior art systems, elimination of near simultaneous events even if those events were valid, was beneficial in that it reduced or eliminated the chance for erroneous position determination of the valid events. It is important to note that any loss in accuracy of valid event position or the addition of event positions resulting from invalid events blur the resulting image by creating background noise. Thus, in the prior art systems, without pulse pile-up rejection, image resolution and contrast was compromised.